Fabrication of III-nitride power device with reduced gate to drain charge

ABSTRACT

A III-nitride power switch that includes a III-nitride heterojunction, field dielectric bodies disposed over the heterojunction, and either gate conductive bodies that do not overlap the top surface of the field dielectric bodies or power contacts that do not overlap field dielectric bodies or both.

RELATED APPLICATION

This is a divisional of application Ser. No. 12/009,045 filed Jan. 16,2008, which itself claims priority to U.S. provisional application Ser.No. 60/881,405, filed Jan. 19, 2007. The disclosures in theabove-referenced patent applications are hereby incorporated fully byreference into the present application.

FIELD OF THE INVENTION

This present invention relates to III-nitride type power switchingdevices and more specifically relates to a III-nitride power switch witha reduced Q_(GD) and a process for the fabrication thereof.

DEFINITION

III-nitride refers to a semiconductor alloy from the InAlGaN system.Examples of III-nitride semiconductors are GaN, AlGaN, AlN, InAlGaN,InGaN, InN, or any combination thereof.

BACKGROUND AND SUMMARY OF THE INVENTION

Referring to FIG. 1, a known III-nitride power semiconductor deviceincludes III-nitride multilayer body 21 formed on a substrate 20.Substrate 20 is preferably formed of silicon, but may be formed of SiC,Sapphire or a III-nitride semiconductor such as GaN. Multilayer body 21includes a III-nitride active heterojunction 21A. Active heterojunction21A includes III-nitride barrier layer 21B (e.g. AlGaN) formed on aIII-nitride channel layer 21C (e.g. GaN). As is well known, thethickness and composition of barrier layer 21B and channel layer 21C areselected so that a two-dimensional electron gas (2DEG) is formed inchannel layer 21C close to the heterojunction of layer 21B and layer21C. The current in the device is conducted through the 2DEG. Note thatIII-nitride multilayer 21 may include a III-nitride transition layer(e.g. formed with AlN), and a III-nitride buffer layer (e.g. GaN layer)disposed between substrate 20 and heterojunction 21A, when for example,substrate 20 is non-native (i.e. is not from the III-nitridesemiconductor system) to the III-nitride system. For example, whensilicon is used as a substrate material.

A device as described above further includes a first power electrode 30(e.g. source electrode) coupled ohmically to heterojunction 21B andsecond power electrode 30′ (e.g. drain electrode) coupled ohmically toheterojunction 21A whereby current may be conducted between electrode30, 30′ through the 2DEG. A gate dielectric body 27 is disposed overheterojunction 21A through which gate conductive body 35 can becapacitively coupled to the 2DEG in order to interrupt (depletion mode)or restore (enhancement mode) the same depending on the type of device.

The device further includes field dielectric bodies 25. Each fielddielectric is disposed between a power electrode 30, 30′ and gateconductive body 35. As illustrated field dielectric body 25 is thickerthan gate dielectric 27. Gate conductive body 35 extends over a fielddielectric body 25. Note that each electrode 30, 30′ also rises alongadjacently disposed field dielectric bodies 25 and over a portionthereof.

In the device shown by FIG. 1, overlap of the gate metal (a) overdielectric 25 contributes to gate to drain charge (Qgd). The overlap (b)of the ohmic electrodes 30 over dielectric 25 increases pitch size.

In a III-nitride device according to the present invention, either theoverlap of gate metal or the overlap of ohmic metal over the fielddielectric bodies, or both, are eliminated through, for example, achemical mechanical polishing (CMP) step. As a result, Q_(GD) of thedevice may be improved, and also the cell pitch of the device may beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-section of a III-nitride device according tothe prior art.

FIG. 2 illustrates a first preferred embodiment of the present inventionin which the ohmic contacts and gate contact terminate in the plane ofthe top of the field dielectric layer.

FIG. 3 shows a second embodiment of a device according to the presentinvention.

FIG. 4 shows a third embodiment of the invention.

FIG. 5 shows a fourth embodiment of the present invention.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2, in which like numerals identify like features asdiscussed above, a III-nitride power switch according to the firstembodiment of the present invention includes gate conductive body 35that does not overlap the top surface of adjacently disposed fielddielectric bodies 25. Preferably, gate conductive body 35 includes a topsurface that is coplanar with top surfaces of adjacently disposed fielddielectric bodies 25. As a result, a device according to the firstembodiment may exhibit a reduced Q_(GD) value.

Furthermore, in a device according to the first embodiment powerelectrodes 30, 30′ do not overlap the top surface of adjacently disposedfield dielectric bodies 25. As a result, the cell pitch of a deviceaccording to the first embodiment may be reduced, which in turn allowsfor reduction of R_(DSON) of the device by allowing a greater number ofactive cells per unit area.

Referring now to FIG. 3, in which like numerals identify like features,a III-nitride switch according to the second embodiment of the presentinvention includes gate conductive body 35 that does not overlap the topsurface of adjacently disposed field electric bodies 25 (and preferablyincludes a top surface coplanar with top surfaces of field dielectricbodies 25), while power electrodes 30, 30′ overlap the top surface ofadjacently disposed field dielectric bodies. Note that in a deviceaccording to the second embodiment, an etch stop layer 50 can bedisposed above gate conductive body 35 after the planarization thereofto protect dielectric bodies 25 and gate conductive body 35 during thefabrication of power contacts 30, 30′.

Referring to FIG. 4, in which like numerals identify like features, adevice according to the third embodiment also includes gate conductivebodies 35 that do not overlap the top surfaces of adjacently disposedfield dielectric bodies (and preferably include top surfaces coplanarwith the top surfaces of adjacently disposed field dielectric bodies25), and at least one power contact 30 that does not overlap the topsurface of adjacently disposed field dielectric bodies 25. Note that adevice according to the third embodiment includes barrier bodies 40disposed under gate conductive bodies 35, and at least on opposing sidesof power contact 30. The application of barrier bodies 40 is disclosedin U.S. patent application Ser. No. 11/702,727, entitled III-nitrideSemiconductor Device, filed Feb. 6, 2007, assigned to the assignee ofthe present application, the entire content of which is incorporated bythis reference. Note that a device according to the third embodimentincludes dielectric lips 25 under barrier bodies 40 that are adjacent topower contact 30. According to the present invention, barrier bodies 40also do not overlap the top surface of adjacently disposed fielddielectric bodies 25, and are preferably coplanar with adjacentlydisposed field dielectric bodies 25.

Referring now to FIG. 5, in which like numerals identify like features,in a device according to the first embodiment of the present invention,power contacts 30, 30′ (note that only one power contact is shown forsimplicity) do not overlap the top surface of adjacently disposeddielectric bodies 25 (and include preferably top surfaces coplanar withtop surfaces of adjacently disposed field dielectric bodies 25), whileconductive gate bodies 35 overlap the top surface of adjacently disposedfield dielectric bodies 25. Note that an etch stop layer 28 can beapplied atop power contacts 30, 30′ after the planarization thereofaccording to the present invention, to protect power contacts 30, 30′and field dielectric bodies 25 during the fabrication of gate conductivebodies 35.

To fabricate a device according to the present invention, an epitaxiallydeposited III-nitride multilayer structure 21 is built atop substrate20, using any desired method. Structure 21 includes at least oneIII-nitride heterojunction having a two-dimensional electron gas (2DEG)to serve as a conductive channel. For example, structure 21 includes aheterojunction between a GaN layer and an AlGaN layer that produces a2DEG layer (not shown) which permits conduction between a source and adrain contact, under the control of a gate structure. Note thatsubstrate 20 may be preferably a silicon wafer, but may also be a SiCwafer, Sapphire wafer, or a III-nitride wafer such as a GaN wafer,without deviating from the scope and the spirit of the invention.

Next, a field dielectric layer 25 is formed atop layer 21 and an activemask step is employed to etch windows 26 in the field dielectric layer25. A thin gate dielectric layer 27 is then formed at the bottom ofwindows 26.

Thereafter, conductive gate metal are deposited inside openings 26 andpatterned to obtain gate conductive bodies 35 inside windows 26 overgate dielectric bodies 27. Preferably, in the same step, windows 29 areopened in field dielectric bodies 25 for the reception of contact metal.Next, contact metal is deposited inside windows 29, and thereafter, in achemical mechanical polishing (CMP) step, the excess metal is removeduntil at least the top surface of field dielectric bodies is reached. Asa result, a III-nitride device is obtained according to the firstembodiment that includes gate conductive bodies 35 and power contacts30, 30′ that do not overlap the top surface of adjacently disposed fielddielectric bodies 25.

Note that a rapid thermal appeal (RTA) can be applied after the CMP stepor before the CMP step. A typical RTA is carried out at 800° C. to 900°C. for 30 seconds to 120 seconds. Thereafter, other features such asgate, source, and drain routing can be fabricated according to anydesired method.

To fabricate a device according to the third embodiment, a device can befabricated as set forth in U.S. application Ser. No. 11/702,727, andthen subjected to a CMP step according to the present invention. Thus,the fabrication of a device according to the third embodiment mayinclude the deposition or otherwise the formation of a barrier body 46after the gate dielectric deposition.

To fabricate a device according to the second embodiment, after thedeposition of a gate metal, a CMP step is applied to obtain a gateconductive body 35 that does not overlap adjacently disposed fielddielectric bodies. Thus, the CMP stops at least at the top surface offield dielectric bodies 25. Thereafter, a thin layer of oxide or nitrideis deposited atop field dielectric bodies 25 and conductive gate bodies35 to serve as an etch stop layer 50. Next, field dielectric bodies 25are patterned to include windows 29 for the reception of contact metal.Next, contact metal is deposited and patterned to obtain power contacts30, 30′ according to the second embodiment.

To fabricate a device according to the fourth embodiment, fielddielectric layer 25 is first patterned to include windows 29 for thereception of power contacts 30, 30′, contact metal is deposited, andaccording to the present invention a CMP step is applied to remove thecontact metal. The CMP preferably stops at the top surface of fielddielectric bodies 25, whereby power contacts 30, 30′ are obtained thatdo not overlap adjacently disposed field dielectric bodies. Next, anetch stop layer 28 (e.g. an oxide or a nitride layer) is applied atopfield dielectric bodies 25 and the planarized power contacts 30, 30′.Thereafter, windows 26 are opened in the field dielectric layer, gatedielectric 27 is deposited at the bottom of windows 26, and gate metalis deposited inside windows 26 and patterned to obtain gate conductivebodies 35 as illustrated by FIG. 5. Note that etch stop layer 28prevents damage to field dielectric bodies 25 and power contacts 30, 30′during the patterning of gate metal to obtain gate conductive bodies 35.

Field dielectric 25 can be a silicon nitride, or silicon oxynitride, ora metal oxide such as Al₂O₃, HfO₂, at a thickness of 500 Å to 5000 Å.Gate dielectric 27 can be silicon nitride or a metal oxide such as SiO₂,Al₂O₃, HfO₂, TiO₂ at a thickness of 20 Å to 500 Å.

The contact metal 30, 30′ include Ti and Al and capping layers such aNi/Au, Mo/Au, Ti/TiW, Ti/TiN.

It should be noted that each III-nitride power semiconductor deviceaccording to the present invention includes a III-nitride multilayerbody 21 formed on a substrate 20. Substrate 20 is preferably formed ofsilicon, but may be formed of SiC, Sapphire or a III-nitridesemiconductor such as GaN. Multilayer body 21 includes a III-nitrideactive heterojunction 21A. Active heterojunction 21A includesIII-nitride barrier layer 21B (e.g. AlGaN) formed on a III-nitridechannel layer 21C (e.g. GaN). The thickness and composition of barrierlayer 21B and channel layer 21C are selected so that a two-dimensionalelectron gas (2DEG) is formed in channel layer 21C close to theheterojunction of layer 21B and layer 21C. The current in the device isconducted through the 2DEG. Note that III-nitride multilayer 21 mayinclude a III-nitride transition layer (e.g. formed with AlN), and aIII-nitride buffer layer (e.g. GaN layer) disposed between substrate 20and heterojunction 21A, when for example, substrate 20 is non-native(i.e. is not from the III-nitride semiconductor system) to theIII-nitride system. For example, when silicon is used as a substratematerial.

First power electrode 30 (e.g. source electrode) is preferably coupledohmically to heterojunction 21B and second power electrode 30′ (e.g.drain electrode) is preferably coupled ohmically to heterojunction 21Awhereby current may be conducted between electrode 30, 30′ through the2DEG. Gate conductive body 35 can be capacitively coupled to the 2DEGthrough gate dielectric body 27 in order to interrupt (depletion mode)or restore (enhancement mode) the same depending on the type of device.

Each field dielectric body 25 is disposed between a power electrode 30,30′ and gate conductive body 35. As illustrated field dielectric body 25is thicker than gate dielectric 27. Note that each power electrode 30,30′ also rises along adjacently disposed field dielectric bodies 25.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein.

What is claimed is:
 1. A method for fabricating a III-nitridesemiconductor switch, said method comprising: forming a III-nitrideheterojunction; forming a field dielectric layer over said III-nitrideheterojunction; etching a window inside said field dielectric;depositing a conductive body inside said window such that saidconductive body fills said window and overlaps with a top surface ofsaid field dielectric outside of said window; removing said conductivebody that overlaps with said top surface of said field dielectric suchthat said conductive body is only disposed inside said window; andcovering said top surface of said field dielectric and said conductivebody inside said window with an etch stop layer, patterning said fielddielectric to form another window therein, and depositing anotherconductive body inside said another window.
 2. The method of claim 1,wherein said conductive body is removed using CMP.
 3. A method forfabricating a III-nitride semiconductor switch, said method comprising:forming a III-nitride heterojunction; forming a field dielectric layerover said III-nitride heterojunction; etching a window inside said fielddielectric; depositing a conductive body inside said window such thatsaid conductive body fills said window and overlaps with a top surfaceof said field dielectric outside of said window; removing saidconductive body that overlaps with said top surface of said fielddielectric layer such that said conductive body is only disposed insidesaid window; and etching another window in said field dielectric afterdepositing said conductive body, and depositing another conductive bodyin said another window prior to removing said conductive body such thatsaid another conductive body fills said another window and overlaps witha top surface of said field dielectric outside of said another window.4. The method of claim 3, wherein removing said conductive body alsoremoves said another conductive body that overlaps with said top surfaceof said field dielectric outside of said another window such that saidanother conductive body is only disposed inside said another window. 5.The method of claim 4, wherein said removing comprises a CMP step.